Patent GB-683820-A (GB)
Title: Improvements in and relating to pulse code signalling systems
Inventor: LEVY MAURICE MOIESE
IPC: H03M1/00
Filed: 1950-02-01
Abstract: 683,820. Multiplex pulse code signalling. GENERAL ELECTRIC CO., Ltd. Jan. 8, 1951 [Feb. 1, 1950], No. 2638/50. Class 40 (v). In a circuit for producing binary code pulse groups from amplitude-modulated pulses, two sets of pulses are used, the amplitudes of which decrease exponentially and the pulses of the first set being interlaced with those of the second set. Each positive amplitude-modulated pulse charges a condenser and a switching device, when operated, applies a negative charge thereto which is dependent upon the pulse amplitudes of the first pulse set, the amplitudes of the pulses of the second set being subtracted from the potential of the condenser when they occur, the resultant voltage, if its sign is positive, operating the switching device during the succeeding pulse of said first set. The code pulses are based on those pulses of said second set which do or do not actuate the switching device. In a decoding circuit, a condenser is charged, by pulses from a set whose amplitudes decrease exponentially, through a switching device actuated by the code pulses. Amplitude-modulated positive pulses to be coded, curve (a), Fig. 2, are applied to the grid of valve 12, Fig. 1, the valve acting as a cathode follower to charge condenser C to a potential S1, curve (d), Fig. 2, corresponding to the pulse amplitude. Condenser C is connected between the anode and control grid of valve 11 which operates as a class-C amplifier and Miller integrator. An exponentially decaying wave, curve (c), applied to the grid of valve 11, through resistance R, provides the two interlaced positive and negative pulse sets P1, P3, P5, P7, P9 and P2, P4, P6, P8, P10. Pulse P1 is prevented from operating valve 11, the latter being cut off by a negative pulse on its suppressor grid fed over conductor 13 from the anode of valve 12 and due to the amplitudemodulated input pulse. The condenser C is also connected to the grid of the conducting triode of a pair 16 having a common cathode resistance for the comparison of its potential with a fixed bias, which may be earth, applied to the grid terminal 17 of the non-conducting triode of the pair. The negative pulse P2 is superposed on the condenser potential and if the resultant does not reach earth potential the comparison triode remains non-conducting, so that the next positive pulse P3 applied to valve 11 results in an integrating action to add a negative charge to condenser C. The potential of the latter is now at point 17, curve (d). In the example shown, another similar operation and comparison with pulses P4, P5, reduces the condenser potential to the point 18. However, pulse P6 sends the grid of the conducting triode of pair 16 negative so that it becomes non- conducting and the other triode becomes conducting for a period to produce a negative pulse, curve (e), at its anode. This pulse is lengthened in a multivibrator 20 which is triggered by the pulse and reset by the next resetting pulse of the train, curve (g), applied to terminal 22. The output pulse, curve (f), is applied to the suppressor grid of valve 11 and prevents pulse P7 from modifying the potential of condenser C which remains at level 19, curve (d). A similar sequence occurs for pulses P8, P9, the potential of condenser C remaining at level 31 which is the same as level 19. Pulse P10 fails to operate the triode pair 16 so that the multi-vibrator does not operate in the fifth code period. The desired code pulses may be derived by generating recurrent groups of five pulses shown in full and broken lines in curve (h) and using a gating valve, to which the negative pulses of curve (f) are applied, to suppress those of the group occurring at the same time as the latter, Fig. 5 (not shown). The waveform applied to the grid of valve 11 may consist of distinct positive and negative going pulses which may be generated separately and subsequently combined. An inverse arrangement may be used in which the subtractions are performed by the positive pulses and the negative pulses are integrated. The valve 11 may be replaced by a three-valve amplifier, the last valve operating in class-C and the condenser C being connected between the last anode and first control grid, Fig. 3 (not shown). In order to avoid uncertainty in the triggering of the multivibrator 20, a train of pulses 28, Fig. 4, may be applied to terminal 17, Fig. 1, instead of a fixed potential. The tips of these pulses lie upon the zero potential line O and the intersection of pulses 28 with a pulse 29 corresponding, for example, to pulse P6, curve (c), Fig. 2, is then substantially independent of the shape of pulse 29. In an alternative method of operation of the coder, the positive peaks only of curve (c) are applied to the grid of valve 11 and the negative peaks, reversed in sense, are applied to terminal 17. Other modifications of the coder circuit are described, using additional diodes to avoid errors due to the grid cut-off characteristic of valve 11 being other than sharp. A similar decoding circuit is described in which the received code pulses are applied to the suppressor grid of valve 11, Fig. 8, and the wave of curve (c) is applied to the control grid. Pulses of the type shown in curve (b) are used to reset the charge on condenser C, being generated in circuit 45 and applied to valve 12 through a short delay network. The resulting negative pulses from valve 12 cut off valve 11 at its suppressor grid while condenser C is charged positively by the pulse at the cathode of valve 12. Each positive code pulse then allows the appropriate negative charge to pass to condenser C and just before it is reset, a pulse from generator 45, applied to terminal 17, cuts off the normally conducting triode of pair 16 and thereby renders the other one conducting to produce, at its anode terminal 43, a pulse whose amplitude is determined by the residual potential of condenser C. The system may be used with signals which are not regularly recurrent like the pulses of curve (a), Fig. 2, in which case a " start " or synchronizing signal is transmitted, with a waveform distinguished from the code pulses. The synchronizing signal may be used to generate the reset pulse. Specification 652,353 is referred to.